Started sculpting before I should've, how can I add a mirror modifier again (without messing things up)?
You'll need to align the central vertices to the object origin:


Set the 3D cursor to the origin (&#8679; ShiftS > Cursor to Selected while in object mode)
Switch to edit mode (&#8633; Tab) and select the central verts
Set the pivot point to the 3D cursor (.)
Scale them to 0 along the mirror axis. e.g. for mirroring along the X axis, press SX0